1. Field of the Invention
The present invention relates to phosphors for use in fluorescent lamps, and more particularly to an efficient phosphor emitting in the green region.
2. BRIEF DESCRIPTION OF THE PRIOR ART
Recently, a renewed interest has developed regarding the use of narrow band emitting phosphors for use in fluorescent lamps to achieve maximum color rendering properties at lumen levels comparable to standard CW and WW lamps. Members of the lamp industry have found that lamps comprising phosphors with narrow band emissions near 450, 540 and 610 lead to extraordinary improvements in color rendering index, luminance, in color discriminating capability, and in preferred rendering of object colors (brilliance). Commensurate with this interest, lamps are being made available on the market which utilize new narrow band green and blue emitting phosphors.
The luminescent material of the present invention is generally expressed by the formula: EQU M.sub.x Ca.sub.1.sub.-2x (Ce, Tb).sub.x SO.sub.4
where M = Li or Na.
In an article by Dixon and Ekstrand, entitled "Thermoluminescence of Rare Earth Activated CdSO.sub.4, SrSO.sub.4 and BaSO.sub.4 ", appearing in Journal of Luminescence 8 (1974) 383-390, North-Holland Publishing Company, a thermoluminescent material was disclosed, generally expressed by the formula CaSO.sub.4 :Tb. However, a thermoluminescent material exhibits an emission spectra or glow curve, only after the material is subjected to an external source of heat.
In an article by Blasse and Pietersen, entitled "Luminescence of Vanadium and Rare Earth Ions in Alkaline Earth Sulfates", appearing in Journal of Solid State Chemistry 11, 148-149 (1974), there is mention of a CaSO.sub.4 phosphor co-activated with vanadium and various rare earth ions, including terbium. However, the emissions from such a phosphor exhibit only rare earth emission lines when excited with shortwave ultraviolet radiation.
U.S. Pat. Nos. 3,764,554 and 3,669,897 deal with various alkaline earth sulfate phosphors activated with divalent europium which emits longwave ultraviolet radiation when excited by a low pressure mercury arc discharge. U.S. Pat. No. 3,650,976 deals with mixed alkaline earth sulfate phosphors activated with various rare earth ions, including mention of cerium, but no mention of terbium, which also emit in the ultraviolet region with little emission above 425 nm wavelength, when excited by X-rays.
By reviewing the results of the prior art, there appears to be a lack of predictability for emission behavior of a particular phosphor host material under different activation and excitation, both as to the wavelength of emission and the emission intensity.